Current semiconductor device trends—higher frequencies, smaller die size, and increased power—are all contributing to increased heat being generated by integrated circuits (ICs). Too much heat can corrupt the IC's data and/or cause it to fail. Conventional methods for addressing heating effects include thinning the IC's semiconductor substrate (semiconductor die) and/or using passive thermal management structures, such as heat sinks and integral heat spreaders (IHS) in IC packages.
Shown in FIG. 1 is a cross-sectional view of an IC package 100 that includes an IHS 116, heat sink 118, and semiconductor die 106. Here, the die 106 is mounted (top-side down) to a substrate 102 so that bond pads on the die 106 can electrically connect with traces on the substrate 102 by way of controlled collapse chip connection (C4) bumps 108 (i.e., the die 106 is flip-chip mounted to the substrate 102). Underfill material 110 occupies regions between the die 106 and the substrate 102, and fillets 112 (i.e. excess underfill material) are formed along the sidewalls of the die 106. Traces on the die side of the substrate 102 electrically connect with traces on the opposite side of the substrate 102 by way of plated-through-holes and/or vias (not shown). The traces on the opposite side of the substrate 102 electrically connect to an external printed circuit board by way of solder balls 104. The package shown here is commonly referred to as a ball grid array (BGA) package.
The die 106 is attached to integral heat spreader 116 by way of a thermal-interface material (TIM) 114. The heat sink 118 can be coupled to the IHS 116 by way of a combination mechanical clamp and thermal grease TIM (not shown). Heat dissipates from the die 106 to the heat sink 118 by way of TIM 114 and IHS 116. During a typical IC package assembly process, die 106 is attached to the substrate 102 (by way of C4 bumps 108 and underfill material 109) and then the combination die 106/substrate 102 are attached to the IHS 116 using the TIM 114. This sequence can present problems for very thin ICs because (1) they are fragile and susceptible to breakage during handling and (2) excess underfill material (fillets 112) can contaminate the bottom-side (IHS-side) of the die 106. This contamination can interfere with the TIM's ability to conduct heat and/or form defects that can contribute to die 106 breakage during the IHS 116 mounting process.
These problems can be avoided by reversing the sequence of assembly (i.e. mounting IHS 116 to the die 106 first and then mounting the combination die 106/IHS 116 to the substrate 102). However, this requires that the TIM (typically an indium solder) have a higher melting point than the C4 bumps (typically a lead-tin solder). One TIM solder material being investigated to replace indium is gold-tin. Gold-tin solder has a higher melting point (MP) than lead-tin solder (the MP of gold-tin solder is approximately two-hundred eighty degrees Celsius and the MP of lead-tin solder is approximately one hundred eighty three degrees Celsius). However, the melting point of the gold-tin solder is so high that coefficient of thermal expansion (CTE) mismatch problems between the die 106 and the IHS 116 during the die 106 and IHS 116 bonding process are intensified to the point where cooling-induced contraction of the IHS 116 can produce stresses that negatively impact the packaged IC's operation. These stresses can, among other things (1) distort the shape of the die 106, thereby changing its physical characteristics and impacting its performance, and (2) induce delamination of the TIM 117, thereby impeding its ability to transfer heat into the IHS 116. Ultimately, these can all affect the packaged IC's yield, speed, and/or reliability.
It will be appreciated that for simplicity and clarity of illustration, elements in the drawings have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Where considered appropriate, reference numerals have been repeated among the drawings to indicate corresponding or analogous elements.